Rare earth metal-based sintered magnets such as R—Fe—B based sintered magnets represented by Nd—Fe—B based sintered magnets are produced from materials which are abundantly available and inexpensive as resources and also have high magnetic characteristics, and thus are used in various fields today. However, because a highly reactive rare earth metal: R is contained, they have the characteristic of being prone to oxidation corrosion in the air. Therefore, a rare earth metal-based sintered magnet is usually put to practical use with a corrosion resistant film formed thereon, such as a metal film or a resin film. However, in the case where the magnet is embedded in a component and used, such as use in an IPM (Interior Permanent Magnet) motor used as the drive motor of a hybrid car or an electric car, or incorporated into the compressor of an air conditioner, etc., the formation of such a corrosion resistant film on the surface of the magnet is not necessarily required. However, naturally, the corrosion resistance of the magnet needs to be ensured during the period from the production of the magnet until embedding in a component.
As mentioned above, a typical example of a method for imparting corrosion resistance to a rare earth metal-based sintered magnet is a method in which a corrosion resistant film such as a metal film or a resin film is formed on the surface of the magnet. However, in recent years, as a simple technique for improving corrosion resistance, attention has been focused on a method in which a rare earth metal-based sintered magnet is heat-treated in an oxidizing atmosphere (oxidative heat treatment) to modify the surface of the magnet. For example, Patent Document 1 and Patent Document 2 describe methods in which an oxidizing atmosphere is created using oxygen, and a heat treatment is performed therein, and Patent Document 3 to Patent Document 7 describe methods in which an oxidizing atmosphere is created using water vapor alone or a combination of water vapor and oxygen, and a heat treatment is performed therein. However, studies by the present inventors have revealed that even when a rare earth metal-based sintered magnet is surface-modified by such a method, sufficient corrosion resistance is not necessarily obtained in an environment where fine dew drops are repeatedly formed on the surface of the magnet due to the fluctuation of temperature and humidity, such as a transportation environment or storage environment where temperature and humidity are not controlled. The studies have also revealed that although the preferred water vapor partial pressure according to Patent Document 3 to Patent Document 7 is 10 hPa (1000 Pa) or more, when a heat treatment is performed in an atmosphere having such a high water vapor partial pressure, a large amount of hydrogen is produced as a by-product of the oxidation reaction that occurs on the surface of the magnet, and the magnet absorbs the produced hydrogen and thus embrittles, causing the deterioration of magnetic characteristics. Therefore, as an improved method for surface-modifying a rare earth metal-based sintered magnet, the present inventors have proposed, in Patent Document 8, a method in which a heat treatment is performed in an oxidizing atmosphere where the oxygen partial pressure and also the water vapor partial pressure of less than 10 hPa, which is regarded as unsuitable in Patent Document 3 to Patent Document 7, are appropriately controlled. Specifically, they have proposed a method in which a heat treatment is performed at 200° C. to 600° C. in an atmosphere having an oxygen partial pressure of 1×102 Pa to 1×105 Pa and a water vapor partial pressure of 0.1 Pa to 1000 Pa (excluding 1000 Pa).